The present invention relates generally to electronic control systems for internal combustion engines, and more specifically to such systems for controlling engine output torque.
In motor vehicles equipped with electronically controlled throttle systems, driving power (engine output torque) is typically controlled in accordance with an engine torque control algorithm which regulates engine output torque based, in part, on commanded throttle position (or percentage). When employing such an engine torque control algorithm, care must be taken to avoid operating conditions in which torque limitations in the vehicle drivetrain may be exceeded. For this reason, motor vehicle transmissions typically have a nominal input torque capacity associated therewith.
As used herein, the term xe2x80x9cnominal input torque capacityxe2x80x9d of a given transmission is defined as the maximum input torque level at which all of the selectable transmission gear ratios can safely operate. While one or more of the selectable gear ratios may be capable of safely operating at higher input torque levels, the nominal input torque capacity corresponds to the one or more gear ratios having the lowest-valued maximum input torque capacity. Setting the nominal input torque capacity of the transmission at this lowest-valued maximum input torque capacity thus insures safe operating conditions for each of the selectable gear ratios. The remaining drivetrain components must also be specified to have input torque capacities which meet or exceed the nominal input torque capacity of the transmission.
A typical engine torque control algorithm is operable to produce an actual engine fueling rate (EFR), which is a function of various vehicle and engine operating conditions including, for example, engine speed, engine load, desired throttle percentage and cruise control status, wherein EFR is limited, under certain operating conditions, by the nominal input torque capacity of the transmission. A fueling system associated with the engine is then responsive to EFR to fuel the engine in accordance therewith.
While all vehicular transmissions are designed with some nominal input torque capacity, the absolute capacity of individual gear ratios of the transmission may vary, with one or more ratios often having a considerable margin of safety. As a specific example, because torque is not being transmitted through active gear meshes, the actual torque capacity of direct drive ratios (i.e. 1:1) commonly exceed the nominal input torque capacity of a given transmission by significant margins. This concept has been recognized by Richardson et al. in U.S. Pat. No. 5,186,081.
Richardson et al. discloses an engine control system which limits supercharger boost pressure based on a comparison between engine output torque and an input torque limit of the selected transmission gear, wherein separate transmission input torque limits are predetermined for each transmission gear and stored in memory. Richardson et al. thus provides for greater engine output torque in certain transmission gears than is possible using engine control techniques based on a single transmission input torque capacity rating.
The Richardson et al. system, however, suffers from a fundamental drawback in that while vehicle performance may be enhanced in certain gear ratios by increasing engine output torque, such a torque increase may well exceed the input torque rating of one or more of the remaining drivetrain components. Thus, while the transmission may be able to withstand increased engine output torque for certain gear ratios, drivetrain components downstream of the transmission may be damaged or destroyed if the input torque ratings thereof are not properly taken into account in the engine torque control algorithm. This concern is heightened in heavy duty tractor truck applications having additional drivetrain components such as one or more auxiliary transmissions, power take-off systems, interconnecting propeller shafts and other interconnecting drivetrain components.
Many electronic automotive control systems determine the currently selected transmission gear by comparing a ratio of engine speed to vehicle speed. Thus, another drawback of the Richardson et al. system occurs when the vehicle is stationary (vehicle speed=0), which produces an invalid gear ratio calculation. The Richardson et al. system does not provide for regulation of engine output torque during an invalid gear ratio condition such as upon vehicle takeoff, which is typically a situation where management of engine output torque would be desirable to protect drivetrain components from excessive levels of torque in the lower gears (higher numeric gear ratios) of the transmission.
What is therefore needed is a control system for regulating engine output torque as a function of input torque capacity of each of the drivetrain components, including the presently selected gear ratio of the transmission. Such a system should take into account vehicle takeoff conditions and provide for corresponding regulation of engine output torque capability. Ideally, such a system should be optionally configured for regulation of engine output torque capability at any time, as needed, or alternatively only under conditions of vehicle deceleration under heavy engine load operation.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, an engine and drivetrain control system is provided for regulating the output torque of an internal combustion engine. The engine produces the output torque in accordance with a fueling rate provided to a fueling system thereof, and the drivetrain includes at least one transmission coupled to the engine wherein the transmission has a plurality of engageable gear ratios, a drive axle and a propeller shaft coupling the transmission to the drive axle. A control computer is operable to execute software algorithms for regulating fuel rate delivery to the fuel system according to, among other vehicle operation parameters, drivetrain input torque limit considerations. According to one preferred technique for regulating engine output torque, a first engine fueling rate is determined which is an engine fueling rate at which the resulting engine output torque is limited by a maximum input torque capacity of a presently engaged gear ratio of the transmission. A second engine fueling rate is determined which is an engine fueling rate at which the resulting engine output torque is limited by a nominal input torque capacity of the transmission. The engine is then fueled in accordance with the larger of the first and second engine fueling rates.
In accordance with another aspect of the present invention, vehicle acceleration and engine operating power are determined. The engine is fueled according to the first engine fueling rate only if vehicle acceleration is less than a predefined acceleration limit and engine operating power exceeds a predefined threshold, and otherwise the engine is fueled according to the second engine fueling rate.
In accordance with still another aspect of the present invention, a third engine fueling rate is determined which is a fueling rate at which the resulting engine output torque is limited by a maximum operating torque capacity of the propeller shaft, and a fourth engine fueling rate is determined which is an engine fueling rate at which the resulting engine output torque is limited by a maximum operating torque capacity of the drive axle. The engine is then fueled according to a minimum of the larger of the first and second engine fueling rates, the third engine fueling rate, and the fourth engine fueling rate.
In accordance with yet another aspect of the present invention, the engine is fueled according to a comparison between the four engine fueling rates described above only if the present gear ratio of the transmission is  corresponds to one of a set of lower gear ratios  gears thereof, and otherwise the engine is fueled according to the larger of the first and second engine fueling rates.
In accordance with still a further aspect of the present invention, determination of the first engine fueling rate is made by sensing a present gear ratio of the transmission. If the gear ratio is a valid gear ratio, the engine fueling rate at which the resulting engine torque is limited by the maximum input torque capacity of the gear ratio is computed and set equal to the first engine fueling rate. If the gear ratio is an invalid gear ratio and the vehicle is stationary, the gear ratio is set to the gear ratio of the lowest gear of the transmission prior to computing the first engine fueling rate. If the gear ratio is an invalid gear ratio and the vehicle is moving, then the second engine fueling rate is retrieved from memory and set equal to the first engine fueling rate. Alternatively, the first engine fueling rate may be broadcast by a computer associated with the transmission onto a communications datalink connected to the control computer.
One object of the present invention is to provide a control system for regulating engine output torque as a function of the input torque capacity of the presently engaged gear ratios of the transmissions as well as the various remaining components of the vehicle drivetrain.
Another object of the present invention is to provide such regulation at all times, or alternatively only when vehicle acceleration is less than a predefined value and engine load exceeds a predefined threshold.
Yet another object of the present invention is to provide such regulation only in the lower gear ratios  gears of the transmission, and otherwise controlling engine output torque only as a function of the maximum input torque capacity of the presently engaged gear ratio of the transmission.
Still another object of the present invention is to provide a series of different engine fueling rates in memory, which fueling rates are selectable in accordance with the drivetrain input torque capacity considerations, or alternatively providing a mechanism by which the transmission provides the various engine fueling rates and/or gear ratio data to the engine control computer.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.